QUANTICO, Va. — For the first time, three women have passed the Marine Corp’s grueling physical test to become infantry officers, potentially paving the way for women to lead Marines into combat.

Two dozen female Marines have tried to pass the test since the course opened its doors to women in 2012.

It is a test of physical and mental endurance, and it is shrouded in secrecy. How these officers respond to chaos, surprise, and pain, instructors here say, is the best way to figure out how capable they might be of leading their fellow Marines into battle.

The results come as the debates within the Marine Corps and on Capitol Hill intensify about whether to officially allow women to serve on the front lines of combat. Unofficially, they have been on the front lines for years, advocates of women in combat point out.

Under an order from the Secretary of Defense, services must open all combat jobs to women by January 2016, or come back with a marked reason – backed by research – as to why they should not do this.

The test Thursday, known as the Combat Endurance Test (CET), is the first hurdle Marines must pass in order to become infantry officers – the quintessential frontline combat job.

Before this week, one woman had passed the CET, but had to drop out of the remainder of infantry officer training, known as the Infantry Officers Course (IOC) with a stress fracture.

The three women who passed the course yesterday are part of a research study for the decision that will come in 2016.

They ​will now go on to try to complete the IOC. If that happens, they will be the first women ever to do so.

“It’s great that three women completed the CET today,” said Maj. Gen. James Lukeman, who just took over as the head of the Marine Corp’s Training and Education Command and walked the course alongside Marines laden with packs. “It reinforced my belief that we’re doing the right thing.”

Now, he added, “I’m anxious to see how they do in the rest of the course.” Major General Lukeman noted the injury suffered by the first woman to complete the CET. “Hopefully, that won’t happen to these three.”

It is a course, however, that will push many of these Marines to their limit.

As 100 potential infantry officers gathered in the middle of the night Thursday, they carried bags packed with 80 pounds of gear, prepared to cover dozens of miles on foot.

Some had Googled the course, to get some clues as to what they might expect.

Other hints came from the items they were told to bring with them: body armor, shovels, headlamps, waterproofing gear, extra fatigues, and a protractor, among dozens of other implements.

What was clear as they received their instructions in the pre-dawn hours was that this was going to the toughest physical test that most of these top athletes – not unaccustomed to pushing their bodies to the limit – had ever undertaken.

And that it was going to be a very long day. (This reporter hiked along with the Marines, although without the heavy pack.)

They were told they would be expected to mask their pain and carry on with dignity. This job of being an infantry officer – the one they were pushing their bodies to the limit to be selected to do – is, after all, about the troops they would be leading to fight, if they were successful.

“If they could have been following you today, would they be proud of what they saw?” Maj. George Flynn, the director of the Infantry Officers Course, reminded them. “Did you lose your composure? Did you panic?”

In other words, Major Flynn said, would the platoon they might one day have the privilege of commanding say, “'Yes, that’s who I want leading me.' Or would they be embarrassed by you?”

That was the point of the day, and, to this end, the candidates were given some pointers by instructors before the stopwatch began. Now, their adrenaline was palpable – but their bodies soon would begin to struggle mightily under the physical demands.

“You will not be sprawled out on the side of the road like something is wrong,” an instructor ordered them. “You’ll be eyes up, focused.”

With that, they were set loose. As daybreak progressed to mid-day, Marines who began by covering miles at a running pace had slowed considerably, many walking gingerly on boot-blistered feet. Others tried to massage cramping hamstring muscles that kept them from moving with any semblance of fluidity.

Their fatigues were soaked, with sweat and in the aftermath of laps in the pool with full gear, treading water as they lifted their packs and rubber M-16 rifles over their heads.

Through the pull-ups – many of them – and an obstacle course that included scaling a 20-foot high rope more than once and making their way over an eight-foot-tall bar, instructors gauged their techniques.

Some of the Marines tried to finesse the course like they see in the movies, Flynn noted, nodding at one muscled male Marine who tried unsuccessfully – but elegantly – to get over the 8-foot-tall steel bar.

As he took a break to shake out his arms, a female Marine came behind him to take her turn. She jumped to grasp the bar, then “chickened armed” her way up, kicking her legs and ultimately pulling them over before dropping to the other side and continuing the course.

As candidates struggled by, instructors reminded them why they were here.

“You’re wearing your weakness on your face!” one instructor yelled at a flushed and sweating Marine who was walking when he was told to run. “No one cares!”

When at last the grueling test came to an end, the candidates gathered to hear their results. Capt. Ty Anthony, an instructor, asked the Marines before him to raise their hands if this was the toughest thing they had ever done.

About half of the hands went up. One hundred Marines had started the day, including six women, and 70 had passed and would be allowed to continue the three-month course.

Captain Anthony acknowledged the 30 who had not passed the course.

“That hurts, watching people you love and care about fail and struggle,” he said. “It is our dream to be infantry officers – our dream to lead – that was on the line today.”

But imagine how it would feel to see the young Marines they might one day be leading struggle, he told them. The test today was “a mirror for the things you’re going to feel in combat.”

That is because it is in the depths of battle and fatigue that the Marines are going to need good leaders most, Flynn told them.

“They’re going to need you when they’ve been in one too many explosions and they’re starting to think they can’t do it anymore and they’ve lost too many of their friends,” he said.

“You’re not here to have Marines calling you ‘Yes, sir’ or ‘Yes, ma’am.’ This is not about you,” he added. He reminded them to have humility in the face of completing this course. The point, he said, is the young Marines they have not yet met – who will be counting on them to lead.